The present invention relates to a protection device which can be used to protect a terminal which can be connected to a local area network which can provide a remote power feed to terminals. The network can be an Ethernet network, for example.
In the conventional way, terminals connected to a computer local area network (for example: personal computers, printers, etc) are supplied with power locally, from the mains. However, it is desirable for some terminals connected to a computer local area network to receive a remote power feed via the connection used to send and receive data. This applies to telephones connected to a computer network, for example.
In an Ethernet network, for example, a connection comprises eight wires. Four of the eight wires constitute two pairs of wires respectively used to send and receive data. Four of the wires are not used. One way to transmit a remote power feed current is to use the four wires that are not otherwise used. Another way consists of connecting the two terminals of a power supply generator in the remote power feed device to respective center-taps of a transformer winding connected to the data receive pair and a winding of another transformer connected to the data send pair; this is referred to as a phantom circuit. At the terminal end, the power supply voltage is provided at respective center-taps of a transformer winding connected to the data receive pair and a winding of another transformer connected to the data send pair.
The jack to which a terminal is connected is generally used in the following manner:                Four of the eight wires are separated into two pairs for respectively sending and receiving data. The terminal includes a transformer having a winding connected to the receive pair and a transformer having a winding connected to the send pair and each of these windings has a center-tap which can be connected to a reference potential via a low resistance.        Four other wires, which are not used, are connected to ground, often via a combination of resistors and capacitors, to eliminate any crosstalk currents induced by the data signals circulating in the first four wires, and to reduce undesirable electromagnetic emissions.        
If a relatively high power supply voltage, for example 48 volts, is applied to this combination of resistors and capacitors, or to the resistors connected to the center-taps of the transformers, the current flowing in the resistors can destroy them. Terminals designed to receive a remote power feed are not subject to this risk.
However, a conventional terminal that is not designed to receive a remote power feed or to withstand the remote power feed voltage (for example a personal computer, a printer, etc) could be connected to the jack at the end of the connection. There is then a risk of damaging the electrical circuits of the terminal.